The use of X-ray technology for providing two-dimensional images of breast tissue for diagnosis of carcinoma or other abnormalities is well known. X-ray imaging has a number of limitations which are universally recognized by radiologists. In particular, X-ray imaging of breast tissue has the inherent limitation that a mammogram provides only a two-dimensional image of a three-dimensional object. Thus, although a potential area of concern may be indicated on a mammogram, the elevation of the subject area within the breast may be uncertain, leading to a biopsy of broader scope than would otherwise be necessary.
As an alternative to conventional mammography, apparatus has been developed that employs ultrasound technology for breast tissue imaging. Ultrasound imaging devices display echoes received from a piezoelectric transducer as brightness levels proportional to the backscattered echo amplitude. The brightness levels are displayed at the appropriate echo range and transducer position or orientation, resulting in cross-sectional images of the object in a plane perpendicular to the transducer emitting face.
It has long been recognized that ultrasound imaging of breast tissue provides some information about internal breast structures that can be obtained only imperfectly, or not at all, with conventional mammography. Thus, there has been a longfelt need to augment the information obtained by mammography with ultrasonic imaging, using either dedicated ultrasound apparatus as described, for example, in Brenden, U.S. Pat. No. 3,765,403, and Taenzer, U.S. Pat. No. 4,434,799, or free-hand techniques, described, for example, in Mendelson, "Ultrasound Secures Place In Breast Ca Management", Diagnostic Imaging, April 1991, pp. 120-129.
A drawback common to the use of both dedicated ultrasonic apparatus and freehand ultrasound examinations to supplement mammography is the inability to provide geometric registration between the mammogram and ultrasound images. This lack of registration results from movement of the breast tissue (and its internal structures) between the compressed shape induced by the mammography apparatus and the shape induced by the dedicated ultrasound apparatus, or in the case of freehand ultrasound examination, direct palpation of the breast tissue.
Conway, "Occult Breast Masses: Use Of A Mammographic Localizing Grid For US Evaluation", Radiology, 181:143-146 (1991) and Brem and Gatewood, "Template Guided Breast Ultrasound", Radiology, 184:872-874 (1992), describe attempts to achieve spatial registration between a mammogram and an ultrasound image by substituting the compression plate, after the mammogram is taken, with a cut-open compression plate that allows insertion of a manually manipulated ultrasound transducer.
The approach described in the above-mentioned articles has several practical drawbacks, including marking the patient's breast with an indelible pen, the increased length of the procedure, the need to have the mammographer present to ensure correct repositioning the patient's breast on the localization grid after the compression plate (used in mammography) is replaced by the cut-open compression plate, and the need for a priori knowledge of where the location of any suspected lesion. Moreover, as noted in the Conway article, even the use of indelible markings on the patient's skin cannot prevent movement of the underlying breast tissue.
In response to longfelt, but unsatisfied, need to provide registered radiologic and ultrasonic image data, the sonomammography methods and apparatus described in co-pending and commonly assigned U.S. patent application Ser. Nos. 08/145,958, filed Oct. 29, 1993, and 08/277,894, filed Jul. 20, 1994, were developed. Those applications describe combined radiologic and ultrasonic imaging apparatus wherein breast tissue remains immobilized while sequentially exposed to both X-rays and ultrasound. Apparatus described as having similar functionality is also disclosed in International Application WO 94/21189.
Applicants have observed, however, that even for apparatus as described in the above applications, wherein the breast tissue remains immobilized during sequential exposure to radiologic and ultrasound imaging, it is still possible to observe regions of the breast where the ultrasound image appears poorly to correlate to the X-ray image.
In view of the foregoing, it would be desirable to provide apparatus and methods for providing ultrasound images of breast tissue that are correlated to an X-ray image of the breast tissue.
It would be still further desirable to provide apparatus and methods capable of correlating geometrically registered X-ray and ultrasound images to provide holographic views of a patient's breast tissue.